The present invention relates to a cold rolled steel strip provided with at least one surface thereof having an excellent capability for forming a phosphate film on the above-mentioned surface, and a process for producing the same.
It is known that a cold rolled steel strip is produced by cold rolling a hot rolled steel strip having descaled surfaces thereof. The cold rolled steel strip needs to exhibit a capability of forming a phosphate film on the surface thereof, a capability of firmly bonding with a paint coating having an excellent corrosion-resistance on the surface thereof and other properties necessary when the steel strip is practically used. In order to impart the above-mentioned capabilities and properties to the cold rolled steel strip, usually, the cold rolled steel strip is finished by a process comprising the steps of surface-cleaning, for example, in an electrolytic degreasing procedure; heating the surface-cleaned steel strip, which has been wound to form a coil, to a recrystallizing temperature thereof or more in a reducing atmosphere formed in a batch type box-shaped annealing furnace; uniformly heating the steel strip at the above-mentioned temperature in the reducing atmosphere for a predetermined time; first cooling, in the reducing atmosphere, the uniformly heated steel strip to a temperature at which the surface of the steel strip is never oxidized; removing the first cooled steel strip from the annealing furnace; second cooling the removed steel strip to a temperature at which no aging occurs on the steel strip and; finally, temper rolling the second cooled steel strip.
The capability of forming a phosphate film on a surface of a steel strip when the steel strip is subjected to a phosphate treatment refers, hereinafter, to a phosphatizing property of the steel strip.
The capability of firmly fixing with a paint coating resistant to corrosion, on a surface of a steel strip when the steel strip is coated with a paint, refers, hereinafter, to a corrosion-resistant paint coating-bonding property of the steel strip.
The above-mentioned conventional finishing process includes a number of steps and, therefore, is complicated and sometimes to troublesome in connecting the steps to each other. Also, the heating, uniformly heating and first cooling procedures are successively applied to the coil-formed steel strip in the box-shaped annealing furnace. These procedures cause the finishing process to be prolonged. Therefore the productivity and economic efficiency of the conventional finishing process are unsatisfactory.
Under the above-mentioned circumstances, in order to improve the productivity and economic efficiency of the finishing process, various attempts have been made to simplify and/or continuously carry out the finishing process.
For example, it was attempted to omit the surface-cleaning procedure for the cold-rolled steel strip, from the conventional finishing process. However, the omittance of the surface-cleaning procedure resulted in the disadvantage in that when a rolling oil was applied onto the surface of the steel strip, it was converted to an undesirable carbonaceous substance and fine steel particles formed on the steel strip surface were converted into undesirable oxide substances containing silicon and/or aluminium, during the finishing procedure. These converted carbonaceous and oxide substances were firmly fixed to the steel strip surface, and resulted in a poor corrosion resistance paint coating-fixing property of the steel strip.
In order to eliminate the disadvantages of the above-mentioned attempt and to shorten the annealing process in the box-shaped annealing furnace, Japanese Patent Application Laid-open No. 53-131915(1978) discloses a solution. That is, when a coil-formed steel strip which had been heated to a predetermined temperature, uniformly heated at the predetermined temperature for a predetermined time, was subjected to a cooling procedure, the steel strip having an elevated temperature of 400.degree. to 450.degree. C., was removed from the furnace, and exposed to the air atmosphere so as to rapidly cool the steel strip. This method was effective for enhancing the productivity in the finishing process. Also, since this method resulted in accelerated oxidation of the steel strip surface, the removal of the oxide layer on the steel strip surface by pickling or surface-grinding the surface before the temper rooling procedure, resulted in concurrent removal of the carbonaceous substance and the fine oxidized steel particle containing undesirable impurities, from the surface of the steel strip. Accordingly, this method was effective for obtaining a cold rolled steel strip having a brilliant surface appearance and, also, for enhancing the productivity of the batch type box-shaped annealing furnace. However, it was found by the inventors of the present invention that the above-mentioned method caused the surface of the resultant cold rolled steel strip to exhibit a poor phosphatizing property. Furthermore, it was attempted to continuously carry out the annealing and cooling procedures in order to produce a cold rolled steel strip having an excellent workability at a low cost. Basically, the purpose of this attempt was to control the thermal history of the steel strip created during the annealing and cooling procedures. Firstly, it was attempted to continuously anneal a steel strip in such a manner that a cold rolled steel strip was heated to a recrystallizing temperature of the steel strip or more, first cooled to a predetermined temperature, overaged in a predetermined range of temperature for a predetermined period of time, and, finally, cooled again to room temperature or another predetermined temperature.
However, in order to effect the above-mentioned procedures, it was necessary to provide a very long line of equipment including a steel strip surface-cleaning apparatus, heating apparatus, uniformly heating apparatus, first cooling apparatus, overaging apparatus, second cooling apparatus, drying apparatus and temper rolling apparatus connected to each other is series. This caused the cost of the equipment to be very high. Therefore, it was necessary to decrease the number of heat cycles to be applied to the steel strip and to decrease the amount of equipment.
In order to meet the above-mentioned needs, various experiments were carried out. First, a conventional heat-emitting tube type furnace for heating the cold rolled steel strip was replaced by a direct heating furnace having an enhanced heat transmission. This replacement was effective for increasing the heating rate for the steel strip so as to shorten the heating time for the steel strip. The direct heating furnace was effective for using the heat generated therein to high efficiency. Therefore, the attempted heating procedure could be carried in high thermal efficiency.
Second, in the cooling procedure, the conventional cooling method in which a cooling gas was jetted to the steel strip, was replaced by a new method in which cooling water or a mixture of water and air was used. This new cooling method was effective not only for shortening the cooling time, but also, for shortening the overaging time. Also, in the case where the water-air mixture is used as a cooling medium, since the cooling rate of the steel strip can be varied in a wide range, it was easy to change the cooling rate in response to the quality necessary for the steel strip, which quality is variable depending on the use of the steel strip. Furthermore, it was possible to stop the cooling procedure applied to the steel strip when the steel strip reached the desired temperature. Therefore, in the overaging procedure, it was possible to omit a re-heating procedure for the steel strip to an overaging temperature. Accordingly, it is expected that the above-mentioned cooling method will be widely used in the practical annealing procedure.
When the continuous annealing procedure was carried out, it was possible to produce the cold rolled steel strip in a high efficiency. However, it was found by the inventors of the present invention that even when the steel strip was continuously annealed in a reducing atmosphere by using a conventional cooling medium such as a cooling gas jet, the resultant cold rolled steel strip exhibited a poor phosphatizing property in comparison with that produced by using the conventional box-shaped annealing furnace. Especially, it was found that the poor phosphatizing activity of the steel strip was caused when a rapid heating procedure, using the direct heating furnace, was combined with a rapid cooling procedure by using cooling water or a cooling air-water mixture. The heating procedure in the direct heating furnace and the cooling procedure by using the cooling water or air-water mixture were carried out substantially in an oxidizing atmosphere. Therefore, during the heating and cooling procedures, the surface of the steel sttip was oxidized. Accordingly, it was necessary to remove the oxidized surface portion in at least one stage of the continuous annealing process. The oxidized surface portion produced in the direct heating furnace could be reduced only at an elevated temperature in an ignition furnace. However, when the steel strip was cooled, the surface of the steel strip was oxidized again, and it was difficult to reduce the oxidized surface in an overaging furnace which works at a temperature not high enough to effect the reduction. Therefore, it cannot be expected to decrease the number or duration of heat cycles. Also, if the reduction of the oxidized surface portion of the steel strip was insufficiently carried out, the resultant steel strip exhibited an unsatisfactory corrosion-resistant paint coating-fixing activity. Therefore, it was necessary to remove the oxidized surface portion from the steel strip before the temper rolling procedure, by means of pickling, polishing or grinding. This necessity caused the resultant steel strip to exhibit the same poor phosphatizing property as that of the steel strip which had been produced by annealing it in a box-shaped annealing furnace and by removing the annealed strip from the furnace at an elevated temperature.
An important use of the cold rolled steel strip is in the body of an automobile. In practical use, the automobile body is sometimes corroded. This corrosion creates a safety problem for the automobile. In order to reduce the corrosion of the automobile body, a single-surface coated steel strip, in which a side surface thereof is coated and an opposite side surface thereof is not coated, is used for making the automobile body. The single surface-coating of the steel strip can be made by an electro-plating method or melt-coating method. When the coating is carried out by using the electro-plating method, the surface of the steel strip is pickled and, then, plated by passing it through an acid plating solution. The pickling procedure and the acid plating procedure cause the non-plated surface of the resultant plated steel strip to exhibit a poor phosphatizing activity.
Also, in the case of the melt-coating method, it is difficult to prevent at least a portion of the surface not to be coated from being coated with a melt of a metal. Furthermore, since the melt-coating procedure is carried out at an elevated temperature, it is difficult to prevent the oxidation of the surface of the steel strip not to be coated. Therefore, after completing the melt-coating procedure, it is necessary to finish the non-coated surface of the steel strip by means of pickling, polishing or grinding. This finishing procedure causes the non-coated surface of the resultant steel strip to exhibit a poor phosphatizing property.
In order to enhance the phosphatizing property of a steel strip, it is known to spray an aqueous suspension of a sodium phosphate-type phosphatizing activity-enhancing agent, onto a surface of a steel strip, after the steel strip is press-shaped and degreased and before the steel strip is treated with a phosphate. Also, it is known to add a small amount of a heavy metal salt to the phosphate-treating solution in order to promote the reactivity of the phosphate-treating solution itself.
However, when the aqueous suspension-spraying procedure is applied, as a pre-treating procedure, to a press-shaped steel strip, the phosphate-treating procedure is prolonged by the addition of the pre-treating procedure. This prolongation causes the cost of the treating equipment and the cost of the procedure to be increased. Sometimes, the addition of the pre-treating procedure to the phosphate-treating procedure is impossible due to the type of the existing phosphate-treating equipment.
When the heavy metal-containing phosphate-treating solution is applied to a surface of, for example, an automobile body which surface includes a plurality of portions thereof different in chemical reactivity from each other, the results of the phosphate-treatment in each portion of the surface is sometimes different from that of the others. In some portions of the steel strip surface, the formation of the phosphate coating excessively occurs. Also, the addition of the heavy metal salt results in an increase in the cost of the phosphate-treating solution. Furthermore, as a method for enhancing the phosphatizing property of the steel strip, Japanese Patent Application Publication No. 46-7442(1971) discloses that the surface of the cold rolled steel strip is coated with 0.2 to 2 g/m.sup.2 of zinc before the phosphate-treating procedure.
However, the paint-coating method used in automobile-manufacturing factories is changing from an anionic electrodeposition method. Therefore, the phosphate-treating agent is changing from hopeite [Zn.sub.3 (PO.sub.4).sub.2 ] type to phosphophyllite [Zn.sub.2 Fe(PO.sub.4).sub.2 ] type. However, the conventional zinc-coated cold rolled steel strip does not always exhibit a satisfactory phosphatizing property when the phosphophyllite type phosphate-treatment is applied thereto. Also, when the cationic electrodeposition procedure is applied to the conventional zinc-coated steel strip, hydrogen gas is undesirably generated on the surface of the steel strip so that the resultant paint-coating layer is ballooned by the hydrogen gas. Therefore, the zinc-coating should not be applied to the steel strip surface when the cationic electrodeposition method is applied thereto.